You can do this in several ways. Apache has some support for this with a
module, but we'll show how Linux can do this for you, and do so for other
services as well. These commands are stolen from a presentation by Jamal
Hadi that's referenced below.

Let's say we have two customers, with http, ftp and streaming audio, and we
want to sell them a limited amount of bandwidth. We do so on the server itself.

Customer A should have at most 2 megabits, customer B has paid for 5
megabits. We separate our customers by creating virtual IP addresses on our
server.